Ali Khosravi

Assessing the Hydro-Mechanical Behavior of Collapsible Soils Using a Modified Triaxial Test Device

Unsaturated loessial Aeolian deposits tend to experience significant volumetric compression when subjected to loading. This behavior is generally attributed to their open, unstable soil fabric and their weak inter-particle bonding forces, which together yield a soil void structure that is susceptible to collapse. The current study examines two possible mechanisms of pore collapse in a loessial soil: pore collapse induced by an increase in net confining stresses under constant matric suction conditions, such as what occurs when a new structure is constructed on top of a collapsible soil deposit, and pore collapse induced by a change in matric suction conditions under a constant net confining stress, such as what occurs when a collapsible soil deposit beneath an existing structure experiences significant wetting due to a large precipitation event. To accomplish this task, an innovative test approach was used to assess the hydro-mechanical behavior of a highly collapsible loessial soil. The test setup incorporates a set of electronic pressure regulators coupled with three electronic pressure sensors to measure and control the applied pressures, and two high-precision digital volume change measuring devices to measure changes in the volume of the specimen and its degree of saturation. Using this approach, “undisturbed” loess specimens are subjected to either isotropic compression at a constant matric suction, or wetting-induced collapse at a constant mean net stress. Results indicate that the hydro-mechanical behavior of collapsible soils is considerably stress-path dependent. For the same values of mean net stress, the deformation measurements of specimens subjected to isotropic compression were often larger than those subjected to wetting-induced collapse. For the isotropic compression tests, it was shown that the soil water retention curve of the soil depends on the mean net stress. Less sensitivity to the mean net stress was observed for specimens subjected to wetting-induced collapse.

Effect of Soil Structure and Disturbance on Hydromechanical Behavior of Collapsible Loessial Soils

Abstract To characterize the role of specimen disturbance and structure in the hydromechanical behavior of collapsible soils, two sets of wetting-induced collapse and suction-controlled triaxial tests were conducted on intact and reconstituted specimens of a loessial soil taken from a loess deposit in Gorgan, a city in the northeastern Golestan province of Iran. The testing approach used an advanced triaxial testing device that was specifically modified to control pressures applied to a soil specimen and to monitor and measure the amount of changes in volume and water content of the soil specimens during testing using highly sensitive digital sensors with an accuracy of ±0.01 cm3. Results of the wetting-induced collapse tests show that the collapse phenomenon in intact loessial specimens was mostly a stepwise reduction in volume rather than a continuous decrease with a constant rate or a sudden drop in volume as water entered the voids. This behavior is believed to be attributed to the nonhomogenous distr...

The Effect of Asperity Inclination and Orientation on the Shear Behavior of Rock Joints

This study investigates the effect of asperity inclination angle and asperity orientation on the shear behavior of rock joints under constant normal loading conditions. The effects of these two rock joint characteristics were investigated by creating artificial rock joints having a regular pattern of triangular asperities that were oriented at different angles in the plane of shear. Large-scale direct shear tests were conducted over a range of normal stresses, on 0.30 × 0.30 m gypsum blocks containing well-mated joints with different asperity orientation and inclination angle characteristics. Experimental results illustrate the importance of considering both the asperity orientation with respect to the loading direction and the applied normal stress when predicting the shear behavior of rock joints. In general, higher normal stresses increased the stiffness of the rock joints in shearing, while a reduction in the shear strength of the rock joints was observed when increasing the asperity orientation angle. The dilation curves indicated the occurrence of both dilation and lateral displacement during shearing. Two different techniques are used to quantify the condition of the joint surfaces: the first approach utilizes the concept of fractal dimension, and the second utilizes the concept of potential contact area. These approaches can be applied in a useful fashion within the framework of existing shear failure criterion for oriented rock joints.

The Hydro-Mechanical Behavior of Infilled Rock Joints with Fill Materials in Unsaturated Conditions

The existence, nature, and frequency of discontinuities in a given rock mass typically govern the overall shear behavior of the rock. The presence of fine materials such as clay and silt within rock joints, whether the product of infilling or natural weathering processes, can have a significant effect on the shear behavior of the rock joint. When assessing the strength of filled rock joints, it is therefore necessary to determine the separate strengths of both the rock and the joint infill material, and to also have a good understanding of the interaction between the two for various joint geometries and levels of infilling. Much of the existing research on the shear strength of filled rock joints has focused on the influence of joint wall roughness and the thickness of the infill material. The purpose of this paper is to synthesize experience from the technical literature with the goal of highlighting the role of unsaturated stress states when predicting the hydro-mechanical behavior of infilled joints with unsaturated infill materials. In this case, due to changes in the degree of saturation of the infill materials, proper prediction of the shear behavior of infilled joints may require incorporating several factors including the unsaturated stress state of the joint, its stress history, any pertinent hardening mechanisms that might be experienced by the unsaturated infill material, and a hysteretic soil water retention curve relationship for the infill material.

The Measurement of Suction Stress Characteristic Curve for a Highly Collapsible Loessial Soil

This paper describes results of an advanced suction controlled triaxial test device on a loessial soil in an attempt to define the constitutive relationship between suction stress and matric suction (Suction Stress Characteristic Curve) for highly collapsible soils. Due to the presence of void spaces with different degrees of collapse potential within the soil matrix, the collapse phenomenon in loess is believed to be a continuous-stepwise reduction in volume rather than a sudden drop during wetting. Due to this unique volume change behavior, the definition of the suction stress characteristic curve for loess may require a soil-specific experimental testing approach capable of making precise and continuous measurements of volume change and water outflow during loading. The testing approach includes a pressure-feedback control loop to provide the required pressure equilibrium in the system in the case of any change in pressure due to pore structure collapse and two independent digitized volume change measurement devices to measure possible changes in soil/water volume during testing. Based on the experimental measurements, depending on the applied unsaturated stress state variables (e.g., net stress and matric suction), and the consequent magnitude of wetting-induced collapse, the soil specimen may show different behavior during shear. For higher matric suctions, soil specimens experience small changes in volume during the suction equilibrium stage and shear strength curves show a dilative and brittle behavior as shearing proceeds. At lower levels of matric suction or higher magnitudes of net stress, a ductile behavior in the shear stress curves is observed and most specimens experience a decrease in volume during shear. Experimental results indicate a unique critical state line for specimens sheared under varying stress state conditions when defining the SSCC using the measured shear strength data in this study.

Stability Analysis of Seismically Loaded Slopes Using Finite Element Techniques

When performing pseudo-static finite element (FE) analyses to assess seismic slope stability, there are two commonly used methodologies: the gravity-induced method and the strength reduction technique. The primary difference between these techniques is in the path that is traveled to bring the soil structure to the verge of failure. In the gravity-induced method, the applied load is increased to reach failure while in the strength reduction technique the strength parameters of the soil materials are decreased until the slope fails. In the current study, these two techniques are briefly introduced and a simple approach for their implementation into the FE computer program ABAQUS is provided. The capability of existing failure criteria for predicting the pseudo-static factor of safety ( F S ) or the critical acceleration ( K C ) is discussed. Contours of maximum principal plastic strain that are determined using the FE method are shown to strongly parallel the factor of safety map that results from LE analyses. Based on the observations that are made in this study, possible concerns regarding the predictive capability of finite element based techniques for assessing the seismic stability of slopes are discussed.